linux-kernel - Re: [linus:master] [block] e70c301fae: stress-ng.aiol.ops_per

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Message-ID: <20250103064925.GB27984@lst.de>
Date: Fri, 3 Jan 2025 07:49:25 +0100
From: Christoph Hellwig <hch@....de>
To: Niklas Cassel <cassel@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Oliver Sang <oliver.sang@...el.com>,
	oe-lkp@...ts.linux.dev, lkp@...el.com, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	virtualization@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	Damien Le Moal <dlemoal@...nel.org>, linux-btrfs@...r.kernel.org,
	linux-aio@...ck.org
Subject: Re: [linus:master] [block]  e70c301fae: stress-ng.aiol.ops_per_sec
 49.6% regression

On Thu, Jan 02, 2025 at 10:49:41AM +0100, Niklas Cassel wrote:
> > > from below information, it seems an 'ahci' to me. but since I have limited
> > > knowledge about storage driver, maybe I'm wrong. if you want more information,
> > > please let us know. thanks a lot!
> > 
> > Yes, this looks like ahci.  Thanks a lot!
> 
> Did this ever get resolved?
> 
> I haven't seen a patch that seems to address this.
> 
> AHCI (ata_scsi_queuecmd()) only issues a single command, so if there is any
> reordering when issuing a batch of commands, my guess is that the problem
> also affects SCSI / the problem is in upper layers above AHCI, i.e. SCSI lib
> or block layer.

I started looking into this before the holidays.  blktrace shows perfectly
sequential writes without any reordering using ahci, directly on the
block device or using xfs and btrfs when using dd.  I also started
looking into what the test does and got as far as checking out the
stress-ng source tree and looking at stress-aiol.c.  AFAICS the default
submission does simple reads and writes using increasing offsets.
So if the test result isn't a fluke either the aio code does some
weird reordering or btrfs does.

Oliver, did the test also show any interesting results on non-btrfs
setups?